PhD Oral Exam - Gabriel Broday, Electrical & Computer Engineering
Power Electronics Interfaces for DC-Microgrids Applications
This event is free
School of Graduate Studies
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
The decentralization of power generation has become a topic of high interest for industry and academia. The integration of stochastic Renewable Energy Sources (RESs) at the distribution level is facilitated by incorporating them into a Microgrid. In this scenario, DC-Microgrids are a good option since many RESs, such as photovoltaic and fuel cell, and energy storage units present DC output characteristics. Also, the efficiency of the DC-DC interfaces tends to be higher than in DC-AC and issues such as frequency regulation, reactive power control and synchronization are avoided. The control of segments of the distribution system as a Microgrid also helps with the deployment of new large loads such as Electric Vehicles (EVs).
However, the intermittent nature of RESs presents a natural challenge for the large scale implementation of DC-Microgrids. Since weather and nature conditions (such as wind, tides, and sunshine) can be rather unpredictable and are uncorrelated with power consumption needs, DC-Microgrids based on RESs must be strongly supported by fast acting Energy Storage Systems (ESSs) to balance supply/demand and assure high power quality to the system. Among these storage devices, Supercapacitors (SCs) have seen a rise in their popularity for power quality improvement in DC-Microgrids. SCs are devices with a high power density and high charge/discharge rates that can be used to provide sudden bursts of power by managing currents with high gradients, acting as dynamic devices to either supply the necessary power or demand extra power within the DC-Microgrid. Thus, the interface of such system requires that both the power converter topology and the control scheme present the right set of features.
Therefore, this PhD research work discusses the main aspects regarding the operation of power electronics converters and suitable control laws considering the characteristics of the mentioned application. These aspects include: the modulation scheme employed, steady-state characteristics of the power converters and modelling/design of a suitable control law.
First, a unified controller for multi-state operation of the traditional 4-switch Bidirectional Buck-Boost DC-DC converter is proposed. It employs a carrier-based modulation scheme with three modulation signals that allows the converter to operate in all four possible states and eight different modes of operation. A mathematical model is developed for devising a multi-variable control scheme using feedback linearization. This allows the design of control loops with simple PI controllers that can be used for all multi-state modes under a wide range of operating conditions with the same performance.
Then, to deal with the limitations presented by the previous converter, a novel bidirectional DC-DC converter based on a Tapped-Inductor (TI) for higher voltage gain at moderate duty cycles is proposed. What is more, the direction of the current in the intermediate inductor of the new topology does not need to be reversed for power flow reversal, leading to a faster action and avoiding singularities in the control law. Besides, it can employ a similar multi-state and multi-variable modulation scheme that eliminates the Right Half-Plane (RHP) zero, common in Boost-type converters. A systematic approach for deriving control laws for the TI current and output voltage based on exact state feedback linearization is discussed. The performance of the proposed control scheme is verified by simulation for a SC-based ESS.